Frequency control



Feb. 6, 1934. J w CONKUN 1,945,544

FREQUENCY CONTROL Filed May 29 1950' 2 Sheets-Sheet l o o o o nn 0 nnno o o o o n i m I 1 m2 fig 2 non nnnnnu um. mu m 6 I l :5 "u o o o 0 o o n n o 0 on. {g

INVENTOR J.W. CON l ATTORNEY FREQUENCY CONTROL Filed May 29, 1950 2 Sheer,s-Sheet 2 TEEN/NH TING- EGE 48 SU IN PED/QNC'E 8 TL/ED 40 (UIT 44 46 TUNED c/eculr INVENTOR J.W. CONKLI ATTOR N EY Patented Feb. 6, 1934 FREQUENCY CONTROL James W. Conklin, Rocky Point, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application May 29, 1930. Serial No. 456,870

7 Claims.

This invention appertains to high frequency electrical circuits and especially to a system wherein a long line is utilized for frequency discrimination and control.

In the copending application of James L. Finch and James W. Conklin, Serial Number 363,660, filed May 16, 1929, on a Frequency control system, a regenerative amplifier or generator of oscillations is disclosed utilizing the combination of one or more electron discharge devices and a long aperiodic line connected from a point in or associated with the output circuit of one device to a point in or associated with the input circuit of that device in order to feed back energy of a frequency for which the line is a predetermined whole number of half wave lengths long and to discriminate against feed back of energy of other frequencies. In order to increase the selectivity of the system, the line is made long so that the percentage of its length occupied by a half wave length of the energy transferred by the line is small. In addition, in order to have the line long and yet occupy a physically small space, it is preferably made in the form of a coil.

Such a line presents several advantages as pointed out in the application referred to in that it may be simply and rigidly constructed, easily tapped along its length, and temperature controlled. Moreover, it affords a means of applying frequency control directly to a high power amplifier used in long distance transmitters, making the amplifier its own generator of oscillations and thereby eliminating what was heretofore necessary, a low power source of high frequency oscillations, frequency multipliers associated therewith and amplifiers for bringing up the frequency multiplied energy to a value suificient to be applied to the high power amplifier. By the use of the line the low power generator of oscillations and the low power circuits associated therewith, which are the sources of most time consuming adjustments and most liable to break down in high power transmitters, are eliminated; and consequently a material saving in initial cost of the transmitter as well as material saving in the operation thereof is effected.

The use of an ordinary shielded coil for feed back purposes, in the apparatus of the character outlined above, I have found, is satisfactory over certain ranges, usually for relatively longer wave lengths. However, when working with shorter wave lengths of the order of a few meters or less, undesirable end effects occur when using coil long line construction. These end effects may be ascribed to the fact that the end turns of the coil do not have the same inductance that the center turns of the coil have because the end turns do not link inductively with as many coil turns as the center turns link with. That is, the center turns link with adjacent turns on each side 0 of it, whereas the end turns link, effectively with the turns on the side of them nearer the center of the coil. Therefore, if surge impedance or inductance is plotted along the length of a cylindrical coil, it will be found that at the ends it is a155,;

minimum, it rises sharply to a maximum a short distance from the ends and continues substantially uniform throughout the remaining length of the coil at the maximum value. Variations in surge impedance or inductance along the length of thecoil give rise to reflections or, what I have termed heretofore, end effects" which prevent proper aperiodic transfer of energy along the line and in some instances may prevent entirely, transfer of energy from the input end of the line to the output end of the line.

To overcome the foregoing difficulty is the prime object of the present invention and I do so by providing a coil having uniform surge impedance or uniform inductance throughout its effective length. The coil built to fulfill this object may assume, according to this invention, various forms. Briefly, it may consist of a coil with overhanging end turns; it may consist of a coil having a concentrated number of turns at its ends; it may consist of a coil having turns of increased diameter at its ends; it may consist of a coil so arranged with a capacity shield that the square root of the ratio of the inductance of the coil to the capacity of the coil per unit of its length is a constant and equals the surge impedance of the coil; or, it may consist of the combination of a coil having its end turns arranged in one or more of the fashions outlined hereinbefore together with suitably shaped shielding therefor in order to produce uniform surge impedance throughout the length of the coil.

Another prime object of this invention is to provide the combination of such a coil with an electron discharge device, preferably of the screen grid type, for regenerative amplification or generation of high frequency oscillations.

While my invention is described with particularity in the appended claims, it may be best understood both as to its method of operation and. structural organization by referring to the accompanying drawings, in which Figure 1 indicates a cylindrical coil having a uniform number of turns per unit of length and no minal 8 thereof.

which may be uniformly shielded throughout its length,

Figure 2 is a graph showing variations in inductance or surge impedance of the coil along its axial length,

Figure 3 indicates a form of coil built in accordance with the present invention wherein turns at the ends of the coil are concentrated,

Figure 4 is a graph showing a desired characteristic for a coil which is obtained according to the present invention,

Figure 5 indicates a coil whose end turns are increased in diameter in order to have a predetermined characteristic according to the principles of this invention,

Figures 6 and 7 indicate coils shield intermediate their ends in a fashion such that the surge impedance of the coil throughout its length is a constant,

Figure 8 indicates diagrammatically transmitting apparatus incorporating the present invention and involving the use of a single electron discharge device, and

Figure 9 is illustrative of the present invention wherein for oscillation generation an even number of electron discharge devices are cascaded.

Turning to the drawings, Figure 1 indicates a coil 2 which may be uniformly shielded by a concentric, longitudinally split metallic shield (not shown) having a uniform number of turns per unit of its length. When such a coil is made substantially an odd number of half wave lengths long,thewavelength taken being thatcorresponding to a frequency which it is desired to transmit, it may be used as shown in Figure 8 for the selective generation of high frequency oscillations.

The coil 2 as shown in Figure 8 is connected between the output terminal 4 of electron discharge device 6 of the screen grid type and the input ter- Biasing anode potential may be precluded from the coil by means of a blocking condenser 10; and, control electrode biasing potential by virtue of a blocking condenser 12. In order to insure unidirectional or aperiodic transfer of energy from the output of tube 6 to the.input thereof, the line 2 is terminated by a resistance 14 equal in value to the surge impedance thereof. To insure feed back solely through the line at higher frequencies, and not through the interelcctrode capacity of the tube 6, a fourth electrode or screen grid 20 is provided.

By virtue of the fact that line 2 is an odd number of half wave lengths long, potential variations of the control electrode 16 and anode 18 of tube 6 are substantially 180 degrees out of phase giving rise, because of the continual transfer of energy by line 2 from the output circuit of tube 6 to the input circuit thereof, to sustained high frequency oscillations of a frequency corresponding to that which the line is an odd number of half wave lengths long. For other frequencies, the line will transfer energies from the plate to the grid of tube 6 at incorrect phase relation for oscillation eneration and hence, for other frequencies, the line acts as a discriminating agent. For a more full and complete discussion of the action of the line 2 both as a means for securing oscillation generation as well as selective amplification reference is made again to the copending application of James L. Finch and James W. Conklin.

By virtue of transformer 22 the primary of which is in series with the source of anode biasing potential, energy may be radiated from any suitradiator. In order to modulate the output of the oscillator, a keying device 26 is provided which, through radio frequency choke coil 28, applies a biasing potential 30 to the grid 16 of tube 6 which allows of oscillation generation during marking. For spacing, potential 32 is applied to the grid of tube 6 which prevents flow of energy through the electron discharge device.

It is to be clearly understood that the present invention is not limited to the circuit shown in Figure 8. For example, an arrangement such as shown in Figure 9 may be used to advantage for transmitting signals. In Figure 9, sound waves impressed upon transmitter 34 alters the voltage drop across resistance 36 thereby modulating oscillations generated by the cascaded electron discharge devices 38, 40. Of course, if desired, as will be apparent to those skilled in the art, voltage variations across resistance 36 may be amplified by any suitable means before being used for variation in grid bias.

Moreover, as shown in Figure 9 other variations may be employed. As the two tubes 38, 40 are cascaded and form an even number of tubes, for correct feed back from the output circuit of tube 38 to the input circuit thereof, line 2 should be an even number of half wave lengths long. In addition, the line, in order to insure aperiodic transfer of energy, need not be terminated by a resistance but may be terminated by an impedance 42, which is, preferably, equal in value to the surge impedance of line 2. Thus, impedance 42 may be a pure resistance or a tuned circuit which, at the desired frequency, offers an impedance equal in value to the surge impedance of long line 2. Coupling between the stages or between the tubes 38, 40 may be accomplished by means of impedance 44 which may be a resistance or preferably a tuned circuit and, output energy may be taken from another impedance 46 of similar character. The output may be radiated by any suitable radiating antenna 48.

For a still further explanation of the operation of the feed back arrangement shown in Figure 9 reference is again made to Finch and Conklin application, Serial Number 363,660.

Now, coil 2 may be an ordinary cylindrical coil having a uniform number of turns per unit of its length and uniformly shielded throughout its length for feed back of energy of relatively longer wave lengths. However, when higher frequencies are dealt with, and the variation in inductance or surge impedance :r as shown in Figure 2, at the ends of the coil becomes a considerable part of a quarter wave length of the winding, where the quarter wave length is that of the energy that it is desired to be transmitted, annoying reflections take place precluding, as already indicated, proper transfer of energy from the output circuit to the input circuit of the electron discharge device or devices which the line couples together.

To remedy this defect, I concentrate or increase the number of turns at the ends of the coil as shown at point 50 of Figure 3. Concentration of the turns may be readily accomplished, for ex- 3 ample, by banking the turns in banks one over the other at the ends of the coil. In this manner a coil having uniform inductance throughout its length and consequently uniform surge impedance throughout its length, having a characteristic as shown in Figure 4 is obtained. Expressed mathematically, the coil turn concentration should be such that the throughout the length of the coil should be equal to the value at the center of the coil as given by value Y in Figure 2 which indicates the surge impedance at the center of the coil.

Figure 5 discloses another form of coil, according to this invention, wherein the end turns 52 of the coil are successively increased in diameter in a fashion such that the throughout the length of the coil equals the surge impedance at the center of the coil. Depending upon the construction of the coil and the shielding, the diameter increase may follow any law, for example, an equation of the first degree or it may be found advisable to have the increase in diameter or area included by each coil turn to follow some exponential law.

The desired characteristic may also be had in a properly shielded coil. Thus, in Figure 6, the shield 54 intermediate the ends of the coil does not extend throughout the entire length of the coil but is out off near its ends. The resulting structure will have a characteristic approximating that shown in Figure 4.

To obtain the desired characteristic and yet shield the coil throughout its length which is desirable, the arrangement shown in Figure '7 may be resorted to. Here the shield 54 is expanded at its ends funnel fashion as shown at 56 so that the surge impedance of the coil throughout its length remains constant.

Any combination of the shielding methods and means described and coil concentrations or increased diameters may be used for providing a path which, according to this invention, should be a whole number of half wave lengths long having uniform surge impedance throughout its length.

Another way of obtaining the desired result is accomplished by choosing tapping point 58 as shown in Figure l on the coil spaced apart a whole number of half wave lengths long the exact number depending upon the system in which the coil is to be used. The overhanging end turns 60 of the coil should then be chosen such that reflection from the open circuited ends thereof causes an effective increase in inductance at the ends of the coil bringing its value at the ends up to value Y at the center of the coil. This arrangement may also be combined with any of the shielding methods and means described or any of the variations in the end turns of the coil as described hereinbefore.

The improved coil when used in any of the combinations shown, for example in Figures 8 and 9, will result in aperiodic transfer of energy at all frequencies and will not produce undesired reflections as is the case with certain coils which, because of their characteristics, prevent at certain frequencies desired aperiodic transfer of energy.

It is to be clearly understood that the line construction described is applicable to all cases of artificial line construction where lines of this general character are to be used. Thus, for example, it may be used advantageously in an oscillation generator of the type described by C. W. Hansell in his copending application Serial Number 408,489, filed October 18, 1929, wherein along line having standing waves produced thereon is used to pull an oscillator into step with the standing waves. In addition, it is not necessary that the coil be made purely cylindrical, for example,

it may be made in prismoidal form, and may have any convenient cross sectional shape.

' Having thus described my invention, what I claim is:

1. In combination, an electron discharge device having an anode, a cathode, and a control electrode, a long line in the form of a coil having an increased number of turns at its ends per unit of length thereof so as to maintain uniform inductance at the ends of the coil, between the anode and control electrode, the length of the line being several half wave lengths long at a desired operating frequency such that for a desired frequency, potential variations at the anode and control electrode of the electron discharge device are substantially 180 degrees out of phase, and, an impedance equal in value to the surge impedance of said line connected between the line and cathode at the output end of said line.

2. In combination, a screen grid tube, and a line a large whole number of half Wave lengths long at a desired operating frequency, the wave length taken being that corresponding to a frequency that it is desired that the tube selectively amplify, between the output and input electrodes of the tube, said line being in the form of a coil having an increased number of turns at its ends so as to maintain uniform inductance throughout the length of the coil and, an impedance equal in value to the surge impedance of said line connected across the output end of said line and the cathode of said tube.

3. A screen grid tube having an input circuit and an output circuit, said circuit being tuned to approximately a desired operating frequency -and a long line long relative to a half Wave length of the operating frequency coupling the circuits to feed back energy from the output circuit to the input circuit, said line being of such a length as to determine the phase of the energy fed back and being in the form of a shielded coil in which the square root of throughout the length of the coil is a constant, whereby energy fed back over said line maintains input and output electrodes of said tube 180 degrees out of phase, and whereby with a slight change in operating frequency of said tube, there is a rapid and large shift in phase at the output end of said line in such a direction as to cause said tube to return to a desired operating frequency.

4. A screen grid tube having input and output circuits, and means for feeding back energy from said output circuit to said input circuit, said means comprising a long line, in the form of a coil having a concentrated number of turns at its ends, several half Wave lengths long at a desired operating frequency, and, an impedance connected to the ouput end of the line and to the cathode of said tube, said impedance having a value substantially equal in value to the surge impedance of said line.

5. In combination, an electron discharge device having input and output electrodes, including a cathode, a circuit tuned to approximately a desired operating frequency coupled to said output electrodes, a circuit connected between said input electrodes, a long line, several half wave lengths long at a desired operating frequency connected between said input and output circuits said line being in the form of a coil having an increased number of turns at its ends per unit of length thereof whereby uniform inductance is maintained at the ends of the coiled line and, an impedance equal in value to the surge impedance of said line connected between the output end of said line and the cathode of said device.

8. Apparatus as claimed in the preceding claim characterized by the fact that means are provided for preventing inter-electrode feedback within said electron discharge device whereby feedback occurs solely along said line as a consequence of which oscillations generated by said device are maintained at a constant frequency corresponding to the length of said line.

7. In combination, an electron discharge device having an anode, a cathode, a grid and a screen grid, a high frequency coil connected between said anode and cathode, said screen grid acting to prevent anode to grid capacity feedback. a long transmission line several half wave lengths long at a desired operating frequency connected between said anode and grid for establishing desired feedback, said line being in the form of a coil having a concentrated number of turns at its ends, and a resistance equal in value to the surge resistance of said line connected between the output end of said line and said cathode whereby said feedback from said anode to said grid is substantially aperiodic.

JAMES W. CONKLIN. 

